I have done 3, here ae my cliffnotes:
1. Have fun otherwise it will be no fun!
2. Have a good diet and make sure you set weekly goals for training. I use endomondo and track all my training.
3. Stretch stretch and then stretch again.
4. I found getting long back to back runs each weekend prior were key for me. Ie 4 weeks before 18 miles, 3 weeks before 20 miles, 2 weeks before 22 miles, then tone it down till raceday.
5. Start getting used to energy bars, shot blocks or whatever works for you to stay energized during the race. Also your stomach has to get used to eating n the go.
6. Hydrate hydrate hydrate day before the race. Also would recommend thermo tabs or something of that sort to keep you from cramping and allow your body to absorb all the fluids.
7. Last but least if you feel like you are hitting a wall just tell yourself just 1 more mile. Eventually you will hit the finish line.
Good luck and enjoy! I just run for fun, not out to break any records and it is truely one of the things i enjoy most in life. Could be the runners high too, lol.